This invention relates generally to scanned display systems such as those utilizing a cathode ray tube display device. It relates more particularly to systems of image enhancement which employ velocity modulation in one direction of scan.
Typically, scan display systems of the type to which the present invention pertains comprise a viewing screen which is sequentially scanned in two orthogonal directions. The intensity of the displayed image is instantaneously controlled by an intensity control signal. When an abrupt change in brightness level is sought to be reproduced, a problem arises due to the finite time required for the intensity control system to effect a change of image brightness level. Because scan continues during this interval, the brightness change in the transition area is gradual, rather than abrupt, in character producing a blurred edge on the depicted image. In a cathode ray tube, for example, of the type used as the display device in television receivers, electronic games and computer readout terminals, one or more electron beams are directed toward a phosphor viewing screen which emits light when struck by high-energy electrons. The electron beam, or beams, are caused to scan in both horizontal and vertical directions by an electromagnetic deflection yoke positioned on the CRT adjacent the electron beam path. Control electrodes within the CRT facilitate voltage control of the CRT beam current which in turn, controls image brightness. The limited bandwidth of the circuitry driving these control electrodes and producing the intensity control voltages gives rise to the above-described edge blurring when an abrupt or instantaneous change in brightness level is sought.
These difficulties have lead practitioners in the art to develop systems which temporarily arrest horizontal scan during attempts to produce abrupt changes of the intensity signal. One such system is set forth in U.S. Pat. No. 2,678,964 which shows a system in which the horizontal scan is velocity modulated with enhancement signals produced by taking various derivatives of the image intensity signal. The basic idea is to stop, or slow, the movement caused by scansion during the brightness transition period allowing time for the intensity control circuitry to change from one intensity level to another. While it is possible to produce such enhancement by combining the enhancement signal directly with the deflecting field used in the primary scanning circuitry, it has been found generally more advantageous to employ separate deflecting means which are responsive solely to the enhancement signal. Most commonly, these individual deflection means comprise either additional electromagnetic windings similar to the deflection yoke or electrostatic deflection plates positioned at an appropriate point along electron beam travel path to influence the electron path.
The above-described enhancement system (often called spot arrest) provides substantial improvement in the apparent resolution of the transition edges of the displayed image in a bandwidth limited system. However, several problems have resulted which have thus far imposed significant limitations on the degree of improvement observed in the displayed image taken as a whole. For example, when the enhancement signal used comprises a simple first derivative of the intensity control signal, a geometric distortion of the displayed image results, caused by a shift in the position of the image transition. This distortion causes white objects to appear narrower than intended and black objects to appear wider than intended. In addition, the relationship between the point at which the scan is arrested by the first derivative enhancement signal and the most visible changes in beam current intensity, causes the "leading" edge of white objects to display a high intensity border yielding a very "harsh" image element.
Some improvement is realized if the enhancement signal having a first derivative of the intensity control signal is combined with an additional component comprising the second derivative of the intensity control signal. However, the improvement realized is limited to large scene components and degradation of small fine-detail scene components not seen in pure first derivative enhancement results. The second derivative enhancement component degrades the ability of the display system to depict very small objects often called "single picture elements". For example, a fine white line scene component surrounded by a dark background will, in response to second derivative enhancement, be erroneously displayed as a pair of fine intense white lines spaced apart by a lower luminance area.
Accordingly, it is a particular object of the present invention to provide an improved method and structure of image enhancement. It is a more particular object of the present invention to provide image enhancement structures and methods which reduce the geometric distortions occurring in scan modulation image enhancement signals.